Bluff body noise control

ABSTRACT

An aircraft noise-reduction apparatus comprise a flow-facing element ( 1 ) and a flow control device ( 2 ) positioned downstream of the flow-facing element ( 1 ). The flow control device ( 2 ) is arranged, in use, to reduce noise induced by unsteady flow downstream of the flow-facing element ( 1 ).

FIELD OF THE INVENTION

The present invention relates to noise-reduction apparatus for use on anaircraft. More particularly, but not exclusively, the invention relatesto a method of reducing noise generated by the interaction of thelanding gear or parts thereof and the air flowing past it during flight,take-off and/or landing.

BACKGROUND OF THE INVENTION

Flow around bodies generates noise, which is detrimental in particularaerodynamic applications for example where low noise emissions are adesign requirement. One such application where the level of noiseemissions is important is in the design of commercial aircraft. Over thepast decades engine noise has been significantly reduced, for example bythe introduction of high-bypass ratio turbofan engines. However,maintaining the minimum engine ground clearance with such high-bypassratio turbo fan engines results in longer landing gear. Thus, Landinggear on commercial aircraft have been identified as major noisecontributors during approach and landing. The design of a landing gearis primarily based on its structural and dynamic function. This complexgeometric design gives rise to unsteady flow which leads to unwantednoise generation.

Fairings have been proposed as a means of reducing landing gear noise.For example, a noise reduction fairing for an aircraft landing gear isdisclosed in WO 01/04003A1. Such noise reduction fairings at leastpartially shield downstream components such as struts stays andactuators from high-speed flow.

Embodiments of the present invention seek to provide improved oralternate noise-reduction apparatus for aircraft. Some embodiments mayalso reduce the noise generated by noise reduction fairings themselves.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided anaircraft noise-reduction apparatus, the apparatus comprising aflow-facing element and a flow control device positioned downstream ofthe flow-facing element, wherein the flow control device is arranged, inuse, to reduce noise induced by unsteady flow downstream of theflow-facing element.

The applicants have found that unsteady flow, around and in the wake ofa flow-facing element can cause a significant contribution to creationof broadband noise. In particular, it has been noted that unsteadyvelocity fluctuations and/or net lift forces generated in the flow maybe a key noise generating mechanism. As such embodiments of theinvention utilise a flow control device downstream of the flow-facingelement to reduce broadband noise.

In particular, the flow control device may be arranged to reduce theflow fluctuations due to relatively large scale flow structures in thewake. For example, the flow control device may be arranged to suppressvortex shedding downstream of the flow-facing element.

Typically, the flow control device is a passive flow control device. Apassive flow control device may be optimised to provide the desired flowcontrol in a particular phase of flight. For example the flow controldevice may be optimised to provide the maximum noise reduction in flowconditions that would occur during approach and landing.

The flow control device may be a splitter plate extending downstream ofthe flow-facing element. Splitter plates (alternatively referred to as“split plates”) are a known means of aerodynamic flow control and havebeen primarily used to modify the separated wake behind cylinders.Splitter plates generally extend from centre-line of the downstream faceof the cylinder.

The splitter plate according to embodiments of the invention may besubstantially aligned with the free stream airflow and may extend in asubstantially radial direction with respect to the structural element.

In some embodiments the splitter plate may comprise a rigid plate. Therigid plate may have a length that is equal to or greater than thestreamwise length of the structural element.

In alternate embodiments the aircraft noise reduction apparatus maycomprise a pneumatic splitter plate. In other words, a jet of air may beblown downstream from the flow-facing element to create an equivalentflow control effect to that of a rigid splitter plate.

The pneumatic splitter plate may comprise an array of nozzles, forexample a series of holes and/or slots. The array may comprise aplurality of holes and/or slots which are substantially aligned alongthe centre line of the flow-facing element. The array may comprise aplurality of holes and/or slots which are distributed along the lengthof the flow-facing element.

The flow-facing element may be an aircraft structural element. Forexample the flow-facing element may be a strut.

Alternatively, the flow-facing element may comprises a fairing, forlocating upstream of a structural element such that, in use, airflow isat least partially diverted away from the structural element, and theflow control device may be provided between the fairing and thestructural element. Such an arrangement may help reduce self-noise whichmay otherwise be produced by the fairing.

Where the flow-facing element is a fairing, the flow control device maybe arranged to reduce recirculating flow between the fairing and thestructural element.

In some embodiments a splitter plate may be arranged such that it isalso adapted to secure the fairing to the structural member.

In alternate embodiments the flow control device may comprises apneumatic splitter plate arranged to provide a jet of air between afairing and structural member.

The structural element may comprise a component of an aircraft landinggear.

A further aspect of the invention comprises an aircraft landing gearcomprising a noise reduction apparatus according to an embodiment of thefirst aspect of the invention.

A further aspect of the invention comprises a method of reducing noisecaused by landing gear on an aircraft including the steps of identifyinga part of the landing that contributes to the noise generated by thelanding gear when in flight, and providing an aircraft noise-reductionapparatus according to an embodiment of the first aspect of theinvention to reduce the noise generated by said part.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying schematic drawings inwhich:

FIG. 1 schematically illustrates, in plan view, a noise reductionapparatus in accordance with an embodiment of the invention whichutilises a rigid splitter plate;

FIGS. 2A and 2B schematically illustrate, in plan view, noise reductionapparatus in accordance with alternate embodiments of the inventionwhich utilise a pneumatic splitter plate;

FIGS. 3A and 3B show the pneumatic splitter plate used in the noisereduction apparatus of FIGS. 2A and 2B; and

FIG. 4 schematically illustrates, in plan view, a noise reductionapparatus in accordance with an embodiment of the invention whichutilises a noise reduction fairing and splitter plate.

DETAILED DESCRIPTION OF AN EMBODIMENT

FIG. 1 shows a noise reduction apparatus in accordance with a firstembodiment of the invention. The noise reduction apparatus comprises astructural element 1 which is exposed, in use, to an airflow V_(∞). Inother words, the structural element is flow-facing. V_(∞) may be assumedto be the free stream airflow. In the case where the structural element1 is a landing gear component it will be appreciated that it may bedeployable, such that it is only be exposed to the airflow V_(∞) duringtake-off, landing and approach.

The structural element 1 is a bluff body, in this case a H-Beam. Theskilled person will appreciate that a bluff body may be generallycharacterised as any body where there is significant flow separation anda generally unsteady wake.

The noise reduction apparatus further comprises a flow control device inthe form of a splitter plate 2. The splitter plate 2 is a rigid plateattached to the downstream side of the structural element 1. Thesplitter plate 2 extends perpendicularly from the downstream surface ofthe structural element 1 and is located on the centre line of theelement.

The splitter plate 2 is arranged such that it is substantially alignedwith the free stream flow V_(∞). When mounted on an aircraft it may beconvenient to simply align the splitter plate 2 with the longitudinalaxis of the aircraft, since this is a reasonable approximation to thefree stream airflow during approach and landing.

Preliminary experiments were carried out to evaluate the effectivenessof this first embodiment of the invention. A H-beam was tested as it isconsidered a good example of a simple bluff-body which produces noiseover a broad range of frequency spectrum. A splitter plate 2 having alength L, measured in the streamwise direction, was attached to the rearof the element 1 having a length W. A selection of different splitterplate lengths (L/W=1, L/W=2 and L/W=3), and a body without a splitterplate, were tested.

A comparison of flow visualisations with and without the presence of thesplitter plate showed that the presence of the splitter plate blockedinteraction between shear layers in the vicinity of the body. The shearlayers continued to converge downstream leading to a longer and widerwake.

The Coefficient of Drag for each arrangement was also compared. Theaddition of the L/W=1 splitter plate resulted in a drop in thecoefficient of drag of C_(d)=0.47. Increasing the length of the splitterplate reduced the drag further by C_(d)=0.23 between L=W=1 and L=W=3.

Standard deviations of velocity plots were used to compare the unsteadyflow. The unsteadiness was concentrated around the H-beam with thehighest velocity fluctuation just aft of it. In the L/W=1 configurationthe unsteadiness moved further downstream and away from the model.

The narrowband spectra were measured in an anechoic chamber and plotscompared for the different configurations to show how the noisesignature of the model was affected. The L/W=0 case showed a strongtonal peak at a Stroudal Number (based upon the width of the body) of0.125 and broadband noise “hump” cantered about a Stroudal Number of0.75. In the cases of L/W=1, L/W=2 and L/W=3 the tonal peak wassuppressed and the noise was reduced over the whole frequency range. Thesplitter plate configurations showed very similar noise spectra up to aStroudal Number of 17.5. Above that frequency the L=W=2 configurationshowed marginally lower noise levels.

Source localization plots were used to identify where origin of thenoise reduction. The comparison between the plots showed that the H-Beamis no longer the main noise source when the splitter plate is used.Rather, the noise source is located towards the trailing edge of thesplitter plate.

FIG. 2A and 2B show the use of a pneumatic splitter plate for downstreamflow control. FIG. 2A illustrates a cylinder bluff body 11 a as aflow-facing element. FIG. 2B shows a H-Beam 11 b flow-facing element. Ineach case a pneumatic splitter plate is provided by means of a blowingdevice 12 a, 12 b on the downstream face of the respective element 11 a,11 b. As with the rigid splitter plate 2 of FIG. 1, the pneumaticsplitter plate is located on the centre line of the element.

As shown in FIGS. 3A and 3B, the blowing device 12 a and 12 b simplycomprises a pipe attached to the rear of the element 11 a, 11 b with aseries of nozzles in form of simple holes 14 or slots 13. The holes 14or slots 13 are distributed along length of the pipe (and therefore,along the length of the element 11 a, 11 b) to form an array.Pressurised air is provided to the pipe to provide blowing from thenozzles in the form of a relatively small jet downstream.

In preliminary experiments carried out to evaluate the effectiveness ofthis second embodiment of the invention, the pneumatic splitter platewas found to provide the same flow effects as a physical split plate.For example the pneumatic splitter plate delays the roll-up of vorticesbehind the element 11 a, 11 b and interrupts the interaction of shearlayers. The noise reduction effect of the pneumatic splitter plate wasalso equivalent to that of the rigid plate. Only a relatively smallblowing rate was required to provide the equivalent effect of the L/W=1splitter plate arrangement of the first embodiment.

FIG. 3 shows a further embodiment of the invention in which theflow-facing component is a fairing 25, positioned upstream of astructural element 21 and arranged to at least partially divert the freestream airflow away from the element 21. Such fairings have beenproposed for noise reduction purposes. However, the applicants haverecognised that in some circumstances the noise-reduction fairing 25 mayitself contribute to the total broadband noise of the aircraft. Thus,according to embodiments of the invention a splitter plate 22 isprovided in the cavity defined between the fairing 25 and the element21. The splitter plate 22 may conveniently be arranged to support thefairing 25 from the structural element 21.

The splitter plate 22 reduces or eliminates vortex shedding from thefairing 25 and in turn reduces noise. As with the previous embodimentsthis is due to the splitter plate 21 blocking the interaction betweenopposing shear layers. The splitter plate 22 also reduced theinteraction between the shear layers and the downstream element 21.

It will be appreciated that the rigid splitter plate 22 mayalternatively be replaced by a pneumatic splitter plate (as describedabove) attached to the downstream side of the fairing 25.

Preliminary experiments were carried out to evaluate the effectivenessof this further embodiment of the invention. Three different sizes ofelements 21 were used to investigate the possibility of reducing thesize of the fairing 25 with respect to the element 21. Aerodynamic andacoustic results were performed in wind tunnel and anechoic facilities.

In the configurations without the splitter plate 22 a recirculatingregion of flow was observed in the cavity between the fairing 25 and theelement 21 as the shear layer aft of the fairings' trailing edgeimpinged on the element part, rolling up inside the cavity. The element21 was subjected to relatively high-speed flow due to the shear layerinteraction.

The application of the splitter plate for the two smaller elements 21blocked the interaction between the opposing shear layers and inhibitedthe shear layer from interacting with the element. As a result therecirculating flow inside the cavity was reduced considerably. Thelarger element 21 was large enough for the shear layer to impinge on it,nevertheless the splitter plate 22 impeded the strong recirculation flowwithin the cavity. Instead a low velocity wake was observed aft of theelement 21. The effect of this change in flow structure had an impact onthe noise produced. The source strength around the apparatus wassignificantly reduced as the magnitude of the velocities and theunsteadiness around the fairing 25 and the element 21 were lower, hencereducing the dipole strength attributed with the fluctuating lift forceson the apparatus. The strong shedding produced a strong tonal peak inthe noise measurements, increasing the overall noise signature. Thesplitter plate reduced or totally eliminated this tone. Theconfigurations involving the two smaller elements 21 reduced this tonalpeak by about 14 dB, measured from the ⅓-octave band spectra. The largerelement eliminated the tonal peak completely although a second smallertonal peak was observed at a high frequency.

The skilled person will appreciate that any of the embodiments of theinvention may be applied to aircraft components as required and may beparticularly beneficial when applied to aircraft landing gear. It mayfurther be appreciated that different embodiments of the invention maybe preferred dependent on the particular application and its associateddesign constraints. For example, in some applications the rigid splitterplate may be preferred due to its simplicity whereas in otherapplications the pneumatic splitter since it may offer space and/orweight savings while offering the same functionality.

Although the invention has been described above with reference to one ormore preferred embodiments, it will be appreciated that various changesor modifications may be made without departing from the scope of theinvention as defined in the appended claims.

For example, it will be appreciated that any of the embodiments of theinvention may be tuned for a particular application. For example thedimensions of a rigid splitter plate or the nozzle size, position andair-pressure of a pneumatic splitter plate may be optimised. Suchoptimisation may take into account a number of design factors, forexample, the level of noise reduction, the aerodynamic benefit (orpenalty), weight and/or space constraints may be considered.

The skilled person will appreciate that the blowing effect of apneumatic splitter plate could be variable in use to provide an activeflow control device. Equally a rigid plate could be partially or fullydeployable to provide an active flow control device.

Equally, similar effects could be used to provide a noise control systemwhich is only activated when required (for example during approach andlanding but not during take-off, when engine noise is dominant)

The skilled person will also appreciated that the blowing device used inthe pneumatic splitter plate embodiment is not limited to an externalpipe arrangement such as those shown in FIGS. 2 and 3. For example thedevice may be formed from a pipe embedded in the structure or may blowair from an internal plenum chamber.

1.-16. (canceled)
 17. An aircraft noise-reduction apparatus, theapparatus comprising an aircraft structural element and a splitter plateextending downstream of the structural element and arranged, in use, toreduce noise induced by unsteady flow downstream of the structuralelement, wherein the splitter plate has a length which is equal to orgreater than the streamwise length of the structural element.
 18. Theaircraft noise reduction apparatus of claim 17, wherein the splitterplate is a passive flow control device.
 19. The aircraft noise reductionapparatus of claim 17, wherein the splitter plate extends in asubstantially radial direction with respect to the structural elementand is substantially aligned with the free stream airflow.
 20. Theaircraft noise reduction apparatus of claim 17, wherein the splitterplate comprises a rigid plate.
 21. The aircraft noise reductionapparatus of claim 17, wherein the structural element comprises acomponent of an aircraft landing gear.
 22. A Landing gear for anaircraft comprising a noise reduction apparatus according to claim 17.23. An aircraft noise-reduction apparatus, the apparatus comprising anaircraft structural element and a flow control device positioneddownstream of the structural element and arranged, in use, to reducenoise induced by unsteady flow downstream of the structural element,wherein the flow control device comprises a pneumatic splitter plate.24. The aircraft noise reduction apparatus of claim 23, wherein thepneumatic splitter plate comprises an array of nozzles.
 25. The aircraftnoise reduction apparatus of claim 23, wherein, the pneumatic splitterplate comprises a plurality of holes and/or slots which aresubstantially aligned along the centre line of the flow-facing element.26. The aircraft noise reduction apparatus of claim 23, wherein thepneumatic splitter plate is a passive flow control device.
 27. Theaircraft noise reduction apparatus of claim 23, wherein the structuralelement comprises a component of an aircraft landing gear.
 28. A Landinggear for an aircraft comprising a noise reduction apparatus according toclaim
 23. 29. An aircraft noise-reduction apparatus, the apparatuscomprising a fairing, for locating upstream of a structural element suchthat, in use, airflow is at least partially diverted away from thestructural element, and a splitter plate provided in the cavity definedbetween the fairing and the structural element and arranged, in use, toreduce noise induced by unsteady flow downstream of the fairing.
 30. Theaircraft noise reduction apparatus of claim 29, wherein the splitterplate is arranged to reduce recirculating flow between the fairing andthe structural element.
 31. The aircraft noise reduction apparatus ofclaim 29, wherein the splitter plate is adapted to secure the fairing tothe structural member.
 32. The aircraft noise reduction apparatus ofclaim 29, wherein splitter plate comprises a pneumatic splitter plate.33. The Aircraft noise reduction apparatus of claim 29, wherein thesplitter plate is a passive flow control device
 34. The aircraft noisereduction apparatus of claim 29, wherein the structural elementcomprises a component of an aircraft landing gear.
 35. A Landing gearfor an aircraft comprising a noise reduction apparatus according toclaim 29.